Opaque chrome coating has been used for many years as a low-reflectance, opaque aperture coating for optical elements, photomasks, and black matrix for LCD displays. Opaque chrome coating typically has three layers: a very thin chrome (Cr) flash for adhesion to a substrate, followed by a chrome oxide (CrOx) layer for low reflection, followed by a thicker chrome (Cr) layer for opacity. The thickness and composition of the opaque (Cr/CrOx/Cr) coating layers are chosen to achieve a desired opacity and low reflectance. Optimal layer composition and thickness may be experimentally determined or derived (P. Baumeister, “Starting designs for the computer optimization of optical coatings,” Appl. Opt. 34(22) 4835 (1995)). Carbon and nitrogen are often added to improve the reflectance and etch resistance of some of the layers (e.g., U.S. Pat. No. 5,230,971 issued to Alpay). More complex opaque chrome coating structures are known (e.g., U.S. Pat. No. 5,976,639 issued to Iwata).
Opaque Cr/CrOx/Cr coating layers are usually deposited on a substrate by a physical vapor deposition technique, typically thermal evaporation, e.g., electron beam evaporation or resistance evaporation, or sputtering. One of the most economical methods for depositing opaque Cr/CrOx/Cr coating layers on a substrate is ion-assisted electron beam evaporation. In general, the method involves sequentially generating vapors of chrome and chrome oxide using an electron beam evaporator and depositing the vapors on a substrate while bombarding the film growing on the substrate with a low energy ion beam. The ion bombardment allows for denser and more uniform films than without ion assist. The more uniform the films, the more consistent the optical properties of the opaque Cr/CrOx/Cr coating. The denser the films, the more resistant the opaque Cr/CrOx/Cr coating is to cracking and pinhole formation. An aperture can be patterned in the opaque Cr/CrOx/Cr coating layers with standard photolithography.
Opaque Cr/CrOx/Cr coating layers deposited with ion-assisted beam evaporation are generally not robust during downstream processing. A simple ultrasonic cleaning of the opaque Cr/CrOx/Cr coating can produce many pinholes in the coating. Patterning of the opaque Cr/CrOx/Cr coating increases the pinhole density in the coating. It is known that chrome typically grows with a columnar structure, which causes tensile stress, (Nakajima, K. et al., Vacuum, 51(4) 761 (1998) and Zhao, Z. B. et al., Journal of Applied Physics, 92(12) 7183(2002)), and that the stress of Cr layers deposited by ion-assisted electron beam evaporation is typically high and tensile. The tensile stress and columnar microstructure are believed to be responsible for the increased pinhole density during patterning. A crack or defect in a film in tensile stress tends to pull apart to release the stress. Water from the aqueous processing steps of the photolithography can enter the cracks and voids between the columnar grains. The shear stress applied to the film during lamination can open up cracks and pinholes.
The robustness of opaque Cr/CrOx/Cr coating downstream processing can be improved by reducing or eliminating the tensile stress in the opaque Cr/CrOx/Cr coating layers. The tensile stress in the opaque Cr/CrOx/Cr coating may be reduced by depositing the opaque Cr/CrOx/Cr coating layers by sputtering or ion-assisted deposition with high DC bias (Nakajima, K. et al., Vacuum, 51(4) 761 (1998) and Zhao, Z. B. et al., Journal of Applied Physics, 92(12) 7183 (2002)). However, experiments show that ion-assisted electron beam deposition with high DC bias cannot fully eliminate the tensile stress in the thicker, top chrome (Cr) layer. The sputtering methods for depositing opaque Cr/CrOx/Cr coating layers are not economical because of the high capital cost of the sputtering equipment—inline or load-locked planar magnetron systems are needed to achieve both high throughput and compressively-stressed opaque Cr/CrOx/Cr coating (Hoffmnan, D. W., Journal of Vacuum Science Technology, 20(3) 355 (1982)).